1. Field of the Invention
This invention relates generally to thermoplastic packaging materials and, more specifically, to flexible, multilayer films.
2. Background of the Invention
Many food products are processed in thermoplastic film packages by subjecting the packaged product to elevated temperatures produced by, for example, immersion in hot water or exposure to steam. Such thermal processing often is referred to as cook-in, and films used in such processes are known as cook-in films.
A food product that is packaged and processed in this manner can be refrigerated, shipped, and stored until the food product is to be consumed or further processed by, for example, slicing and repackaging into smaller portions for retail display. Alternatively, the processed food can be removed immediately from the cook-in package for consumption or further processing (e.g., sliced and repackaged).
A cook-in film must be capable of withstanding exposure to rather severe temperature conditions for extended periods of time while not compromising its ability to contain the food product. Cook-in processes typically involve a long cook cycle. Submersion in hot (i.e., about 55.degree. to 65.degree. C.) water for up to about 4 hours is common; submersion in 70.degree. to 100.degree. C. water or exposure to steam for up to 12 hours is not uncommon, although most cook-in procedures normally do not involve temperatures in excess of about 90.degree. C. During such extended periods of time at elevated temperatures, any seams in a package formed from a cook-in film preferably resist failure (i.e., pulling apart).
Following the cook-in process, the film or package preferably conforms, if not completely then at least substantially, to the shape of the contained food product. Often, this is achieved by allowing the film to heat shrink under cook-in conditions so as to form a tightly fitting package. In other words, the cook-in film desirably possesses sufficient shrink energy such that the amount of thermal energy used to cook the food product also is adequate to shrink the packaging film snugly around the contained product. Alternatively, the cook-in film package can be caused to shrink around the contained food product prior to initiating the cook-in procedure by, for example, placing the package in a heated environment prior to cooking.
Some cook-in applications impose some very stringent performance requirements on films for use therewith. For example, some food products that are processed via cook-in procedures are oxygen sensitive. Cook-in films for these products need to include one or more oxygen barrier layers. Other cook-in applications require that the film or the package made therefrom be printable and be able to retain any image printed thereon.
An increasingly important requirement of cook-in films is that they have good interply adhesion. This is complicated where a layer derived primarily from a homo- or co-polymer of propylene is to be adhered directly to a layer derived primarily from a homo- or co-polymer of ethylene. Although ethylene and propylene are homologues, polymers made from one tend not to adhere well to polymers made from the other. One attempt to counteract this tendency toward poor adhesion has involved blending a polymer including mer units derived from propylene and having a low Vicat softening point with a homo- or co-polymer of ethylene so as to increase the compatibility of the layer formed therefrom with the layer derived primarily from a homo- or co-polymer of propylene. However, even where such a film as made exhibits good interply adhesion, that same film can exhibit mediocre or even poor interply adhesion after it is oriented.
Further, when films of this type are sealed, the sealing process can induce delamination between the seal layer and the layer adjacent thereto. During the cooking process, these same two layers must provide the film with structural integrity and support the seal formed in the seal layer. Also, the seal layer needs to be able to resist the degenerative effects of grease and/or fatty products which often are encountered during cook-in processes. Thus, the need remains for an oriented film with good interply adhesion and sealability.